Pharmacology. Labetalol is a mixed alpha- and beta-adrenergic antagonist; after intravenous administration, the nonselective beta antagonist properties are approximately sevenfold greater than the alpha1 antagonist activity. Hemodynamic effects generally include decreases in heart rate, blood pressure, and systemic vascular resistance. Atrioventricular conduction velocity may be decreased. After intravenous injection, hypotensive effects are maximal within 10–15 minutes and persist for about 2–4 hours. The drug is eliminated by hepatic metabolism and has a half-life of 5–6 hours.

Indications. Labetalol may be used to treat hypertension accompanied by tachycardia associated with stimulant drug overdose (eg, cocaine or amphetamines) and clonidine withdrawal. Note: Hypertension with bradycardia suggests excessive alpha-mediated vasoconstriction (See VII. Hypertension, See Pseudoephedrine, Phenylephrine, and Other Decongestants); in this case, a pure alpha blocker such as phentolamine (See Phentolamine) is preferable because the reversal of beta2-mediated vasodilation may worsen hypertension. In addition, it may have an unpredictable effect on coronary vascular tone; other agents, such as nitroglycerin, may be preferable for stimulant-induced coronary vasoconstriction.

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